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04 Forester X, MT
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Discussion Starter #1
OK, my 2004 manual says 87 is OK for NA but you have to have 91 for the Turbo.

Going by memory, I believe the NA is 8.5:1 compression ratio and the turbo is 10:1?

Would using 87 in my NA give any benefits?

Would it burn more thoroughly and help the cats last longer?
 

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2006 Forester XT
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Low octane has nothing to do with burn rate or combustion speed.

It's a widespread myth that high octane gas burns slower or less completely and/or leaves deposits.
 

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2007 Forester Sports XT 4EAT
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The XT engine has a lower compression ratio than the N/A engine. Higer octane fuel is required to prevent detonation. There is a good article here. :smile:

Bobby...

My MODding Journal
 

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Discussion Starter #4
so i must have remembered it backwards?
lower compression for turbo, higher for NA?
better check the manual.

i meant to ask would using the higher 91 octane in a NA car that only requires 87 be of any real benefit?
 

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so i must have remembered it backwards?
lower compression for turbo, higher for NA?
better check the manual.

i meant to ask would using the higher 91 octane in a NA car that only requires 87 be of any real benefit?
It might be able to advance the timing a little bit, but nothing really noticeable. Possible to get slightly better gas mileage. I don't think running anything over 89 will make any noticeable difference though.
 

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2007 Forester Sports XT 4EAT
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Other than taking a larger bite out of your wallet, you're not going to hurt your N/A engine by burning 91 octane fuel. IMO, if the higher octane fuel makes your N/A engine run better, either the 87 octane fuel was poor quality, or your engine may need some maintenance. :confused:

You could split the difference & use 89 octane fuel. :wink:

Bobby...

My MODding Journal
 

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2015 Highlander AWD XLE 6AT
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Possible to get slightly better gas mileage. I don't think running anything over 89 will make any noticeable difference though.
Agreed. Another option is to mix one gallon of 91 to every three gallons of 87 to make your own 88 AKI. If you have 93 available instead of 91, then use a 1:5 mix.

HTH,
Jim / crewzer
 

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2004 Forester XT Premium 4EAT
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This: High Octane

The swami has been hearing a lot of nonsense around the gas pumps these days. People are tanking up with the "good" stuff because the commercials imply that it's better for their engine. When the oil companies use superlatives like "Super", "Extra" and "High"...well it must be better, right? And of course they wouldn't be charging $0.10 - $0.20 more unless they were putting some really good stuff in there, right? Sorry...NOT!

"High Octane" is not synonymous with "good" or "better", and does not mean that it is better for your engine! And the chances are pretty good you don’t need high octane fuel in your scooter.
High-octane fuels only become necessary when your engine has a high compression ratio. It’s a very long and complicated story…that the swami will make short.

First important fact that you must accept:

All gasoline, regardless of its’ octane rating, have pretty much the same amount of energy per gallon. What!!! "Sacrilege" you say? Well, actually, some higher-octane fuels have a few LESS percent energy per gallon…so as not to argue over this small point, for the sake of this discussion we will all agree that the automotive gasoline that you buy at the pump, regardless of octane rating, has the same amount of potential energy.

Second important fact that you must accept:

Octane is NOT a measure of power but of the fuels’ resistance to ignition from heat. A higher-octane fuel, under identical combustion chamber conditions, will burn slower.

How can this be? If all of the above is true, how do we get more power out of high octane gasoline? We do, don’t we?

Well…yes we do. Here’s how:

But first you must understand "heat of compression". There is a 2,000 year old fire starting device that still amazes the swami. A length of bamboo was hollowed out leaving one end capped. A stick, about the same length as the bamboo, was whittled down until it fit snugly into the bamboo cylinder. A bit of dried grass or wood shavings were placed in the bottom of the bamboo cylinder and the snugly fitting stick was violently rammed down the bamboo tube. The heat generated from rapidly compressing the air in the tube was sufficient to ignite the tinder.

The same thing can happen in the cylinder of an engine. The piston, quickly squeezing the fuel/air mixture into a small space, can generate enough heat of compression to ignite the fuel well before the spark plug fires, with unpleasant results. If the fuel prematurely ignites while the piston is on its way up, the burning of the fuel, in conjunction with the rising piston, creates even more pressure, resulting in a violent explosion. This explosion is equivalent to hitting the top of the piston with a very large hammer. If you want to be able to see through the top of your piston, ignore those sounds that are usually called: "pre-ignition", "ping" or "engine knock". Trust me on this one; in his reckless youth, using this method, the swami turned a few pistons into paper weights.

What we really want is a very rapid burn of the fuel, not an explosion. And we want the burning of the fuel to take place while the piston is in a better position to convert this pressure into productive work, like on its way down. Think of this burning as a very fast "push" on the top of the piston. Despite the violent noises you hear from some exhaust systems, it really is a rapid push on the top of the piston making the crankshaft go around, not explosions.

So that we can ignite the fuel at exactly the right time with the spark plug, instead of from the heat of compression, they put stuff into gasoline to keep it from igniting prematurely. The more resistant the fuel is to ignition from the heat of compression, the higher its octane rating.

Are you with me so far?

Higher compression ratios = higher combustion chamber pressures = higher heat… and it is with these higher combustion chamber temperatures that the magic happens.

At higher temperatures the fuel is burned more efficiently. So, while it’s true that the higher-octane fuel does not posses any more energy than low octane fuel, the increased octane allows the extraction of more of the potential energy that has always been there. Conversely, lower compression ration engines utilize a little less of the fuel energy potential (2-4% reduction) but there is also less heat generated in the combustion process.

So how do you know if you need high-octane fuel? The swami suggests you look in the owners’ manual! Manufacturers really do want you to get the maximum efficiency out of your engine. They do their best to give a good balance between horsepower and engine life. It’s in their best interests to do so.

There is ABSOLUTELY NO BENEFIT to using a higher octane than your engine needs. The only benefit is increased profits to the oil companies that have cleverly convinced some of the public that their new "Super-Duper, Premium-High-Test, Clean-Burning, Used-By-Famous-Racing-Types-All-Around-The-World, Extra-Detergent-Laden-Keep-Your-Pipes-Clean, Extra-High-Octane" fuel is your engines’ best friend. The swami is telling you the truth, don’t listen to that talking cartoon car.

The swami hears people insisting that they got better mileage, better acceleration, and less dental plaque by switching to a high-octane fuel. The swami reminds these people that in every pharmacy is a special miracle pill that is often prescribed by doctors, it works wonders because people believe that it works wonders; it’s called a "placebo". The swami warns: never confuse faith with physics!

If you are getting pinging or knocking with what should be the correct octane for your engine, start by checking the ignition timing, also check that the spark plug is the correct heat range. For 2-strokes, check for excessive carbon build-up on the top of the piston, the carbon takes up space and increases the compression ratio.

If all is well and correct, and you still are getting knocking, then try the next higher octane. You won’t go faster, you won’t go farther, but you will prevent an unsightly hole in your piston.

This subject is a whole lot more complicated than the swami wants to bother with. If you are curious to know more, put some of these words into your search engine and enjoy the education:

Antiknock Index

Octane

Stoichiometric Combustion

Thermal Efficiency

Flame Front

Highest Useful Compression Ratio

Compression Ratio

Placebo
 

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This subject is a whole lot more complicated than the swami wants to bother with.
Interesting... That's an awfully long write-up on a subject with which the "swami" does not want to bother.

This has long been a touchy subject, and Subaru, among other car companies, isn't much help. For example, the spec sheet for the UDM 2010 Forester states the "fuel requirement" for the N/A mtor is "unleaded gasoline (87 octane)". The implication is nothing higher and nothing lower.

However, my '09's user manual states:

Fuel:
Non-turbo models
Use only unleaded gasoline with an octane rating of 87 AKI or higher.
And to really mess with us, the Australian specs for the 2010 Forester state the fuel requirement to be 90 - 98 RON (= ~86 - 94 AKI), and the car's fuel consumption figures are based on using 95 RON (= ~91 AKI) fuel.

This is an issue that I've been tracking in detail for 10 years. Our old 97 Plymouth Grand Vogager (3.3 L V6, 8.9:1 compression ratio) did not benefit from using >87 AKI fuel. My MY2000 Ford pickup (5.4 L V8, 9.0:1 CR) is spec'd for "87 AKI minimum". Performance and fuel economy is dramatically better with 88 - 89 AKI fuel, but there's no improvement above 89 AKI.

My '09 Forester (2.5 L H4, 10:0:1 CR) does better with 88 - 89 AKI fuel in the summer, but an AKI of >87 does not seem to offer any improvement in the winter.

:icon_wink:
Jim/ crewzer


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Interesting... That's an awfully long write-up on a subject with which the "swami" does not want to bother.

This has long been a touchy subject, and Subaru, among other car companies, isn't much help. For example, the spec sheet for the UDM 2010 Forester states the "fuel requirement" for the N/A mtor is "unleaded gasoline (87 octane)". The implication is nothing higher and nothing lower.

However, my '09's user manual states:



And to really mess with us, the Australian specs for the 2010 Forester state the fuel requirement to be 90 - 98 RON (= ~86 - 94 AKI), and the car's fuel consumption figures are based on using 95 RON (= ~91 AKI) fuel.

This is an issue that I've been tracking in detail for 10 years. Our old 97 Plymouth Grand Vogager (3.3 L V6) did not benefit from using >87 AKI fuel. My MY2000 Ford pickup (5.4 L V8) is spec'd for "87 AKI minimum". Performance and fuel economy is dramatically better with 88 - 89 AKI fuel, but there's no improvement above 89 AKI.

My '09 Forester does better with 88 - 89 AKI fuel in the summer, but an AKI of >87 does not seem to offer any improvement in the winter.

:icon_wink:
Jim/ crewzer


(Click on image for details)
The only way to really know would be to datalog. Up the octane until you get to max timing advance. It really depends how much range you have in the ecu's programming.

The other difference is some companies have different additives for the different octane rated fuels. They can use stuff like ethanol to bump up the octane, but it will give you worse fuel mileage.
 

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The only way to really know would be to datalog. Up the octane until you get to max timing advance. It really depends how much range you have in the ecu's programming.
That's my intent, and it's part of the reason why I bought this stuff and posted this report.

I just haven't had a chance to set up any new / long runs and log the data. I need to get my oldest son (a Porsche mechanic) to help me with this...

Some day soon, I hope...
Jim / crewzer
 

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Discussion Starter #12
Can anyone comment what compression other engines are running these days?

10:1 seems a little high not to use 89 or so?! Engine computers are much better these days adjusting timing to compensate and avoid knock so maybe this makes all the difference now?

Now with the 8.5:1 range I would be OK only using 87 (with a non-turbo engine). I presume the reason Subaru turbos need 91 is that they run hotter combustion temps?
 

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You can't just look at static compression to figure out what octane to use, other engine design details will dictate that. Compression changes as engine operates, sometimes will bleed off due to valve overlap in an N/A car and in a boosted car you will have higher cylinder pressure under boost. Hondas run high compression engines and do well on 87 octane.

I'd put 87 in an N/A car as long as there is no knock. I wouldn't put anything less than 91 octane in a boosted car.

Stan
 

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Discussion Starter #14
one thing to think about and ask around about: does octane have any effect on catalytic converter life? i was told by a shop that the cars they get with converter problems almost always run regular 87 fuel.

i wouldn't think it would have an effect on cat life but as a previous owner of 2000 Foresters with P0420 issues and cats replaced earlier than they really should, maybe the extra $ for fuel is worth it?
 

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What about for towing?

I've often wondered about this question, specifically regarding high load conditions such as towing a trailer.
Some other vehicles that require 87 octane say in their manual to use 91 octane when towing.
Does anybody know the specific reasoning for this? Would it matter in a Forester? I'm wondering because I may be towing in the future.
 

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Some other vehicles that require 87 octane say in their manual to use 91 octane when towing.
Does anybody know the specific reasoning for this?
My Ford pickup requires 87 AKI "mimimum", but it generally runs better on 89, especially when towing. Several possible explanations come to mind:

1) Higher load (i.e., towing) typically means higher engine block- and head temperature. The additional load is not trivial, as it not only includes the weight of the trailer, but also an increase in aerodynamic drag.

Operating on 87 AKI, engine performance can suffer as the EMS retards timing to reduce knock in the hot engine. 91 AKI can allow the engine to operate closer to optimal by reducing the load-induced tendency to knock.

2) Many tow vehicles use automatic transmissions, and the AT tends to shift down under load (i.e., climbing a hill) when the timing advance reaches its limit. Higher AKI fuel can allow for additional advance, thereby reducing AT downshifting and gear-hunting. This is where my truck (5.4 L V8, 9.0:1 CR) sees the biggest performance improvement when using ~89 AKI instead of 87.

3) People tend to (recreationally) tow in the summer when air temperature and humidity are relatively high as opposed to the winter, when air temperature and humidity are relatively low. Higher air temperature and/or humidity can lead to a higher AKI requirement.

Would it matter in a Forester?
I would expect so, and other than perhaps a couple of Dollars, there would be nothing lost by experimenting with this if/when you tow.

I don't plan to tow with my Forester (nothing substantial, anyway). However, my experience so far suggests that my car seems to benefit from slightly higher AKI fuel (~88 or so) in the summer, but otherwise appears to operate just fine on 87 AKI in the winter.

HTH,
Jim / crewzer
 

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Low octane has nothing to do with burn rate or combustion speed.

It's a widespread myth that high octane gas burns slower or less completely and/or leaves deposits.
absolutely right!
 

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BTW, crewzer, I never thanked you for your very well written and helpful response. :Banane35:

We'll be towing this May, so I'll experiment with different octanes at that time and post any notable changes in the towing forum.
 

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MG,

My pleasure. I'll be curious about the results of your "experiments"!

Safe travels!
Jim / crewzer
 

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I just did a trip from Kingston, Ontario to Memphis (about 2,620++ miles). I drove down using 87, on the way back switched to 89 . Gas is so cheap in the states, it make sense to do the experiment there.

I took out the mixed city highway driving (which averaged about 20.1MPG)

Best gas mileage was 31.3 using Esso 87 from Canada.
89 was the worst at 20.1 MPG. There is something odd about that though...it is a large dip considering I was averaging about the same speed.

The only variables are that I had the alternator replaced as it started to make an very loud noise (sounded like a bad bearing) on the way down. So I had a new alternator put in for the way back up. Also there was more slightly more luggage on the way back. Including some Schlafly's beer :)

87
31.3 MPG (not using AC) Esso
25 MPG Sunoco
25.3 MPG BP
27 MPG BP
Average 27.15 MPG

89
25.9 MPG BP
20.1 MPG (just remembered there was a long traffic jam, 10 miles of doing stop...go bumper to bumper traffic on the highway, no workers present for the weekend lane closure outside of Indianapolis. In fact on the whole trip didn't see a single worker doing work, 4 standing around...but no doing work) BP
26.8 MPG (not using AC) BP
24.3 MPG Sunoco

Average 24.275 MPG
 
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